Diesel soot photooxidation enhances the heterogeneous formation of H2SO4

Both field observation and experimental simulation have implied that black carbon or soot plays a remarkable role in the catalytic oxidation of SO2 for the formation of atmospheric sulfate. However, the catalytic mechanism remains ambiguous, especially that under light irradiation. Here we systematically investigate the heterogeneous conversion of SO2 on diesel soot or black carbon (DBC) under light irradiation. The experimental results show that the presence of DBC under light irradiation can significantly promote the heterogeneous conversion of SO2 to H2SO4, mainly through the heterogeneous reaction between SO2 and photo-induced OH radicals. The detected photo-chemical behaviors on DBC suggest that OH radical formation is closely related to the abstraction and transfer of electrons in DBC and the formation of reactive superoxide radical (•O2−) as an intermediate. Our results extend the known sources of atmospheric H2SO4 and provide insight into the internal photochemical oxidation mechanism of SO2 on DBC.

some quantitative numbers e.g. how much sulphuric acid can be produced e.g. in atmospheric conditions with different BC concentrations. This can then be compared to gas phase production of sulphuric acid. This discussion has been added into the revised manuscript (Lines 148158).  Q2. line 123, it is said that SO2 concentration is in low level (1 ppm). It is very high concentration; typical atmospheric concentrations are well below 1 ppb.
A2. In China, the average SO2 concentration was in the range of 50 -75 ppb from 2012 to 2014. 3 In the last few years, the average SO2 concentration has exhibited a constant decrease due to the reduction in the emission of SO2. The field observation found that the hourly average concentration of SO2 was in the range of 2-12 ppb during the period of winter residential heating in Beijing (Fig. R3). Thus, to simulate the heterogeneous conversion of SO2 on DBC surface under close to real atmospheric conditions, photooxidation experiments of low-level SO2 (10 ppb and 60 ppb) on DBC were carried out in the flow reactor. Fig. R4A shows the total uptake of 60 ppb SO2 on particles under different conditions. When SiO2 particles in the tube plug flow reactor were exposed to SO2 (60 ppb), SO2 concentrations decreased slightly and returned to the initial concentration in 8 min. This indicated that the reaction of SO2 on the fresh SiO2 surface is weak. In contrast, a sharp decrease in the SO2 concentration was detected in the DBC/SiO2 system under either dark conditions or light irradiation. The uptake process lasted much more than 10 h. Under light irradiation, the total SO2 uptake on DBC is slightly greater than that under dark conditions. These results indicate that the irreversible uptake of SO2 on DBC is significant even under conditions close to the real atmosphere. To further quantitively assess the heterogeneous conversion of SO2 on DBC, extracted SO4 2ions were analyzed using IC (Fig. R4B). As can be seen, the SO4 2concentration is the highest (3.7±0.59 μg ml -1 for 60 ppb SO2) in the presence of DBC under light irradiation compared with the control experiments. This further proved that both light irradiation and DBC have a significant enhancing role on the heterogeneous conversion of SO2. Moreover, the higher SO4 2concentration obtained at 10 ppb SO2 (1.5±0.76 μg ml -1 for 10 ppb SO2) further highlights the enhancing role of DBC photooxidation at lower SO2 levels.  This discussion has been added into the revised manuscript (Lines 133-147).

Q3.
In the main text there are plenty of acronyms, which are not explained like TEMPO in line 182 A3. The acronyms in the main text have been explained in the revised manuscript.
The corresponding full names for these acronyms have been added in the "Methods" section (Lines 428-431).

Responses to Comments from Reviewer #2:
This is an interesting new contribution aiming at unravelling the chemistry that leads to SO2 photoconversion on soot under atmospheric conditions. This is an important topic and is of wide interest, especially in Asian megacities subject to intense haze episodes.
Answer: Thank you very much for giving us these valuable comments and suggestions.
The manuscript has been carefully revised according to your comments (See below).
The manuscript stays quite qualitative and some time vague.

Q1.
For instance, what is meant with "internal catalytic mechanism" (Line 34), or "expose" (line 100, where it is probably a mis use of this word), "inorganic sulfate" (line 132, H2SO4 is inorganic). It is clearly shown that SO2 reacts on the surface of soot, even if the actual speciation of the products is only made indirectly, and that light induce the formation of transient oxidants.

A1.
Thank you very much for pointing out the mistakes in our writing. These mistakes or typos are corrected as follows.
These errors have been corrected in the revised manuscript.
Q2. The reasoning leading to the conclusion that OH is a key player in the current observations is unclear to this reviewer. O2could certainly play a similar role, while OH may be scavenged by the OC fraction on soot, the superoxide may react with both NO and SO2 (and not so much with the alkanes and aromatics). Any thoughts on this?
A2. We agree with you that O2may play a role in these reactions on DBC. Previous studies showed that •O2 − can interact with SO2 or sulfite ion (SO3 2-) to form a series of sulfur-containing radicals (•SO3 -, •SO5 -, and •SO4 -). [4][5][6] These radical chain reactions dominated by sulfur-containing radicals would eventually result in the formation of sulfate. However, it should be noted that these sulfur-containing radicals are scarcely detected in the BMPO spin-trapping ESR spectra of SO2-aged DBC ( Fig.3A) according to the reported measurement methods. 7,8 Thus, we concluded that the contribution of heterogeneous reaction between SO2 and O2to form H2SO4/sulfate should be limited despite the fact that this reaction may happen in this reaction system. This discussion has been added into the revised manuscript (Lines 248-256).

Q3.
It is stated that the irradiation takes place above 350 nm, but there is no mention how the Xenon is filtered to achieve such wavelength. Please add this info. This is indeed a critical point. In fact, such wavelength region limits substantially the nature of compounds that may absorb light and trigger the discussed photochemistry. It certainly means that the long chain saturated alkanes are not involved in the discussed chemistry.
It also means that most of the simple aromatics are not triggering the observations. Which contradicts several statements made in this manuscript concerning the suggested mechanism. Maybe the author should elaborate a bit more on the actual compounds that may indeed act as an electron source (i.e., reaction 1). Fig. R5A. The wavelength region of the Xenon spectrum is in the range of 330 to 850 nm. Without filtering, light with wavelengths less than 330 nm was not detected. The light in the near-infrared and infrared bands was filtered using a transmission-reflection filter (VISREF). This information was added in the revised manuscript (Lines 354-358).  Several studies reported that carbonaceous materials excited under light irradiation can induce the formation of surface electron-hole pairs, especially for these surfaces with plentiful defects and oxygen-containing functional groups. [9][10][11] Given that various oxygen-containing functional groups (Fig. R6) and defects or disordered structures (D peak in Fig.R7) are ubiquitous on carbonaceous materials in DBC, it seems reasonable that the formation of photo-generated holes on excited EC may extract electrons from OC and subsequently donate electrons to other available acceptors such as adsorbed O2. [12][13][14][15] To further verify this assumption, the photo-induced electron-hole pairs induced by the illumination of EC were analyzed using TEMPO spin-trapping ESR spectra. As shown in Fig.R8, the remarkable decrease in the signal intensity of the TEMPO radical after 120 min light irradiation demonstrated that EC in DBC samples can indeed induce the generation of holes or electrons (Eq-1). Thus, long-chain saturated alkanes in DBC could react with photo-induced holes on excited EC (Eq-2) and subsequently be oxidized to other organic oxygen-bearing compounds (Eq-4). 16 The electrons

A3. (1) The Xenon spectrum is shown in
contributing to the formation of •O2were the photo-induced electrons (Eq-3), which could also be considered to be indirectly derived from OC.
The reaction mechanism has been corrected as follows:    This has been added into the revised manuscript (Lines 193-199).  A3. Thank you for your reminder, we have noted the work by Yao et al. 21 Recently, Yao et al observed a good correlation between SO3 formation and traffic-related soot during the early morning in Beijing. 21 They further proposed that the surface catalytic oxidation of SO2 on the ether group sites of soot surfaces may be the crucial contributor of SO3 according to the DFT calculation results by He et al. 22 Based on the IC results in Fig. R4B, it could be found that H2SO4 produced on SiO2/DBC under dark conditions (2.0±0.10 μg ml -1 ) was greater than that in the control experiments (0.35±0.02 μg ml -1 for fresh SiO2/DBC mixture). This indicated that the heterogeneous conversion of SO2 on DBC can also promote H2SO4 formation under dark conditions. Here, we think that this catalytic process under dark conditions could also occur on DBC. Nevertheless, the production of sulfuric acid under light conditions is significantly greater than that under dark conditions (Fig.R4B), indicating that the oxidation of SO2 by photoinduced radicals should be the main route of sulfuric acid formation in the photochemical reactions. This has been added into the revised manuscript (Lines 140-144, and Line 318-321).
Q4. Line 123: Atmospheric SO2 levels are a few ppbv. In the experiments, "low SO2" was about 1 ppm, i.e. about a factor of 1000 higher. Is the heterogenous SO2 conversion that slow that such high SO2 is needed? What does it mean for the reaction rate for atmospheric conditions and its importance for global SO2 oxidation?
A4. Generally, high-level SO2 in in-situ DRIFTS experiments is necessary to easily observe the remarkable discrepancies among different experimental conditions and further explore the reaction mechanism. We agree with you that atmospheric SO2 levels are a few ppb. In China, the average SO2 concentration was in the range of 50 -75 ppb from 2012 to 2014. 3 In the last few years, the average SO2 concentration has exhibited a constant decrease due to the reduction in the emission of SO2. Even so, the hourly average concentration of SO2 was still up to ~12 ppb during the period of winter residential heating in Beijing (Fig. R3). Thus, to simulate the heterogeneous conversion of SO2 on the DBC surface under close to real atmospheric conditions, the photooxidation experiments of low-level SO2 (10 ppb and 60 ppb) on DBC were carried out in the flow reactor (Fig.R1). Fig. R4A shows the total uptake of 60 ppb SO2 on particles under different conditions. When SiO2 particles in a tube plug flow reactor were exposed to SO2 (60 ppb), SO2 concentrations decreased slightly and returned to the initial concentration in 8 min. This indicated that the reaction of SO2 on the fresh SiO2 surface is weak. In contrast, a sharp decrease in SO2 concentration was detected in the DBC/SiO2 system under either dark conditions or light irradiation. The uptake process lasted much more than 10 h. Under light irradiation, the total SO2 uptake on DBC was slightly greater than that under dark conditions. These results indicate that the irreversible uptake of SO2 on DBC is significant even under conditions close to the real atmosphere. To further quantitively assess the heterogeneous conversion of SO2 on DBC, extracted SO4 2ions were analyzed using IC (Fig. R4B). Clearly, the SO4 2-      Or can we draw any special conclusion for this system here?
A5. An important special conclusion on the formation of OH radicals can be drawn from this work. The most recent work published in Angew. Chem. (2022, 134, e2022016) reported that the reaction between water and O2 on carbonaceous soot surfaces under light irradiation can give rise to the formation of gaseous OH radicals via triggering the formation and conversion of singlet O2 ( 1 O2). 17 In this study, our experimental results proved that the transmission of photo-induced electrons and the conversion of superoxide ion (O2 -) on the DBC surface could also contribute to the formation of OH radicals. Thus, our work further complements or improves the production mechanism of OH radicals on DBC under light irradiation and further confirmed its environmental significance.
These discussions have been added into the revised manuscript (Lines 307-314).
Q6. All in all, it is a nice piece of work in the field of heterogeneous SO2 oxidation.
But I see not special novelty value or it is not clearly communicated. If the authors mean that the described process is important for atmospheric SA production, gas-phase SA measurements for close to atmospheric conditions should be added to verify this hypothesis. The manuscript in its present form is more suited for a special journal dealing with heterogeneous catalysis in the atmosphere or atmospheric chemistry generally.

A6.
We appreciate your affirmation of our work. In fact, inspired by the valuable comments and suggestions of all reviewers, we reconsidered the significance of this research. We think this research has implications for the wider scientific community in the following aspects.
Firstly, we verified the formation of gas-phase SA at close to atmospheric conditions.
As mentioned in A1 (2), this study provides direct evidence for the possible existence of gaseous sulfuric acid, especially under light irradiation. This is quite meaningful for further understanding the formation and growth of new particle in the atmosphere.
This discussion has been added into the revised manuscript (Lines 315-327).
Secondly, the conventional view is that BC particles mainly act as a reducing agent.
Fox example, BC is considered to play an important role in the heterogeneous reduction of NO2 to HONO in the atmosphere. 1,23 In contrast, in this work, we first reported that DBC can act as an oxidation medium to directly promote the heterogeneous oxidation of SO2 to H2SO4 by OH radicals. The current three-dimensional air quality models usually underestimate the concentrations of sulfate due to unknown formation pathways. [24][25][26] Thus, our work may provide new insight into the long-standing puzzle regarding an unidentified surface oxidation channel of SO2 .
This discussion has been added into the revised manuscript (Lines 300-305. Thirdly, the photochemical aging of SO2 could change the optical properties and climate effect of BC. It is well accepted that black carbon has strong effects on regional and global climates due to the remarkable positive (warming) radiative forcing. [27][28][29] The change of mixing state caused by aging in the atmosphere is an important reason for the uncertainty in the prediction of the radiative forcing of black carbon. [30][31][32] This study confirmed the possibility of mixing black carbon and sulfuric acid in the atmosphere.
Thus, future study on the optical properties of BC internally mixed with sulfuric acid is necessary for better evaluating the climate effects of aged BC. Moreover, mixing with acidic sulfuric acid will also greatly increase the health risk of BC.
This discussion has been added into the revised manuscript (Lines 331-339).